This application claims priority under 35 U.S.C. xc2xa7119 to Japanese Patent Application No. 2001-219280 filed on Jul. 19, 2001.
1. Field of the Invention
The present invention relates to a zinc oxide polycrystal thin film semiconductor member formed on a silicon substrate, and more particularly, to a semiconductor member ideal as a light emitting element.
2. Description of the Related Art
Recently, it is expected to apply a zinc oxide (ZnO) thin film to a transparent conductive film, a surface elastic wave device, a near-ultraviolet emitting element, and the like. In particular, a single crystal ZnO thin film grown on a single crystal sapphire substrate hetero-epitaxially exhibits excellent electric/optical characteristics, and the application thereof to various devices is positively proceeded. However, the employment of the expensive sapphire substrate makes it non-realistic to apply the thin film to a device requiring a large area. If the ZnO thin film can be grown on a silicon substrate in place of the sapphire substrate, it is possible to make a novel electronic device by combining an ultra-micro processing technology for silicon with the physical properties of the ZnO thin film such as piezoelectricity, ferroelectricity, near-ultraviolet emission, and the like, in addition to the cost reduction of the device.
Hitherto, when it is intended to grow a ZnO thin film on a silicon substrate hetero-epitaxially, a natural oxide film (SiO2) on the surface of the silicon substrate prevents the epitaxial growth of the ZnO thin film. In addition, it is difficult to form a single crystal ZnO thin film of good quality due to an adverse affect such as the reduction of crystallinity that is caused by the difference of the lattice constant.
Similarly, in the study of superconducting oxides, there is almost no example in which a single crystal superconducting material was grown on a silicon substrate epitaxially. In contrast, many reports describe that CaAs, SiC, and the like were grown on a silicon substrate epitaxially, and most of these reports describe that thin films were grown under an ultra-vacuum environment. An amorphous oxide film is formed on a silicon substrate. Thus, unless it is removed, epitaxial growth reflecting the crystallinity of the substrate cannot be performed.
An ordinary method of removing an oxide film is to sublimate it by heating a substrate to at least 1000xc2x0 C. in an ultra-vacuum environment. Unless this method is performed in an ultra-vacuum of at least 10xe2x88x928 Torr, the clean surface of a silicon substrate becomes polluted with oxygen atoms slightly existing in a vacuum environment and the surface of the substrate is oxidized again. Another method of removing the oxide film is to form a clean surface on a substrate by etching the surface of the substrate with hydrogen fluoride, preventing the oxidation of the surface by bonding hydrogen to the resultant clean surface, and removing the hydrogen from the surface by heat of several hundreds of degrees. However, it is also difficult to perform this method in an environment other than an ultra-vacuum environment.
From the reasons described above, in an oxide thin film forming method of forming a thin film by supplying oxygen gas to a silicon substrate the surface of which is made clean, it is apparent that amorphous oxide covers the surface of the silicon substrate and makes epitaxial growth difficult.
A method of forming an zinc oxide thin film on a silicon substrate was reported first by M. Shimizu et al in a report xe2x80x9cGrowth of c-Axis Oriented ZnO Thin Films with High Deposition Rate on Silicon by CVD Methodxe2x80x9d (Journal of Crystal Growth 57 (1982) 94-100). In this report, a zinc oxide (ZnO) thin film was grown on a silicon substrate by means of chemical vapor deposition (CVD). In this method, an n-type zinc oxide was formed on a substrate having a p-type silicon (111) surface so as to make a pn-junction device. The thus obtained ZnO thin film could secure a rectifying property as an electric characteristic and was excellent in crystallinity, in addition to the rectifying property. As shown in FIG. 1, however, in photoluminescence (PL) characteristics obtained by Hexe2x80x94Cd laser, emission of visual light is dominant and light emission in the vicinity of 380 nm that corresponds to the forbidden band width of a ZnO thin film cannot be confirmed at all. Light emission in the vicinity of 380 nm performed by photoluminescence is an important factor for determining whether or not impurities exist in a forbidden band, the composition in a thin film, and whether a structure is good or bad. When the emission of visual light is dominant in photoluminescence as shown in FIG. 1, it suggests that a defect level exists in a forbidden band due to impurities, a defective structure, an abnormal composition, and the like, which is a serious problem when the ZnO thin film is applied to a light emitting element, and the like.
Recently, a method of forming a ZnO thin film on a silicon substrate was found by A. Miyake et al (refer to xe2x80x9cGrowth of Epitaxial ZnO Thin Film by Oxidation of Epitaxial ZnS Film on Si (111) Substratexe2x80x9d (Jpn. J. Appl. Phys. 39 (2000) L1186). In this method, a ZnO thin film was formed by forming zinc sulfide (ZnS) on a silicon substrate and annealing the zinc sulfide in oxygen. A result obtained could be satisfied in a certain degree as to all of crystallinity, composition, photoluminescence characteristics. In particular, the photoluminescence characteristics were such that light emission corresponding to a forbidden band width also could be confirmed while light emission in a visible light region also was observed. Although it is conceived from the above result that this method of creation can be sufficiently satisfied, it becomes apparent that it is difficult to add p- and n-type impurities. It is conventionally conceived difficult to add p- and n-type impurities to a ZnS thin film, and it is needless to say that the addition of the impurities is difficult even if the ZnS thin film is oxidized. Further, it is reported in many papers in the past that p- and n-type impurities must be added to a compound of Group II-VI in a thermal non-equilibrium state. It is conceived difficult to add impurities b the method of A. Miyake et al because a ZnO thin film is formed by thermally oxidizing it at high temperature.
As described above, it is very difficult to form a zinc oxide thin film on a silicon film. A greatest cause of it resides in that a silicon oxide film that is stably formed on a silicon substrate is amorphous. To epitaxially grow other material on a silicon substrate, first, the bonding hands of silicon atoms must chemically bond to the bonding hands of a material to be deposited. Next, it is necessary that the lattice intervals of of these elements be offset a few percentages.
The amorphous silicon oxide film acts as an obstacle to satisfy the above conditions. As described above, many research and development facilities have tried to develop a technology for removing a silicon oxide film from a silicon substrate and for epitaxially growing other material on the resultant surface of the silicon substrate. However, it is very difficult to develop a technology for growing an oxide thin film after silicon oxide is removed, and even if epitaxial growth can be performed unexpectedly, it is difficult to grow an oxide thin film with good reproducibility.
Accordingly, an object of the present invention is to form a polycrystal zinc oxide semiconductor member that is excellent in crystallinity and in composition on a silicon substrate. It is another object of the present invention to add n-type impurities to a thin film formed, to obtain a pn-junction device making use of the a p-type silicon substrate and an n-type ZnO thin film and to arrange the pn-junction device as a light receiving element and a light emitting element.
To achieve the above objects, a zinc oxide semiconductor member of the present invention includes a zinc oxide thin film formed on a substrate having a single crystal silicon surface, wherein the crystal orientation of the surface of the zinc oxide thin film exhibits the c-axis orientation surface of a wurtzite structure, and photoluminescence spectrum when Hexe2x80x94Cd laser (325 nm) is irradiated to the zinc oxide thin film emits light in the vicinity of a forbidden band width in the vicinity of 387 nm.
The zinc oxide thin film may be oriented only to the c-axis of a zinc oxide crystal (002) surface on a silicon substrate surface (111) and may include a zinc oxide buffer layer, and the crystal surface on the buffer layer may rotate 30xc2x0.
After hydrogen is bonded to the surface of the substrate having the single crystal silicon surface by a hydrogen fluoride treatment, a buffer layer may be formed on the substrate by depositing zinc oxide thereon by sputtering, and a zinc oxide thin film may be formed on the buffer layer by depositing zinc oxide thereon by chemical vapor deposition using acetylacetone zinc.
A pn-junction light receiving element may be constructed by constructing a pn junction device the spectral sensitivity of which is higher in a near violet region than in a visible light region by arranging the silicon substrate as a p-type and the zinc oxide semiconductor member as an n-type. Further a pn-junction light emitting element may be constructed the wavelength of light emitted thereby is changed by an increase in an injected current in a forward bias state in which the p-type silicon substrate of the pn-junction device is set to a positive potential and the zinc oxide semiconductor member is set to a negative potential.
As a light emitting element, a hole insertion layer may be inserted into the interface of junction between the p-type silicon substrate and the n-type zinc oxide semiconductor member from the p-type silicon substrate.
Further, the present invention also includes a method of manufacturing the semiconductor member and the elements. The manufacturing method includes the steps of bonding hydrogen on the surface of the single crystal silicon substrate by a hydrogen fluoride treatment, depositing, after the hydrogen fluoride treatment, zinc oxide on the single crystal silicon substrate by sputtering, and depositing a zinc oxide thin film on the deposited zinc oxide by chemical vapor deposition using acetylacetone zinc.
When the single crystal silicon substrate is formed as a p-type and the zinc oxide thin film is formed as an n-type, at least one of acetylacetone metals of Group III is used as a impurity material for forming the zinc oxide thin film as the n-type.